A Concept Plane using electric distributed propulsion Evaluation of advanced power architecture M. Ridel (1), B. Paluch (2), C. Doll (1), D. Donjat (1), J. Hermetz (1), A. Guigon (3), P. Schmollgruber (1), O. Atinault (3), P. Choy (3), C. Le Tallec (3), O. Dessornes (3), T. Lefebvre (1) 1 : ONERA, The French Aerospace Lab, Toulouse, France michael.ridel@onera.fr, carsten.doll@onera.fr, david.donjat@onera.fr, jean.hermetz@onera.fr, Peter.Schmollgruber@onera.fr, Thierry.Lefebvre@onera.fr 2 : ONERA, Lille, France, bernard.paluch@onera.fr 3 : ONERA, Paris, France, antoine.guigon@onera.fr, olivier.atinault@onera.fr, philippe.choy@onera.fr, claude.le_tallec@onera.fr, Olivier.Dessornes@onera.fr, Abstract Starting from electrical distributed propulsion system concept, the ONERA’s engineers demonstrated the viability of an all electrical aircraft for a small business aircraft. This paper describes the advanced power architecture considering energy conversion and power distribution. The design of this advanced power architecture requires the multi-physic integration of different domains as flight performances, safety and environmental requirements (thermal, electric, electromagnetic). From this analyse, all components of this advanced power architecture have been identified and evaluated. Introduction In 2011, an ONERA/CEAtech team has been constituted to meditate on the all electric aircraft (AEA) challenge. One of the main team’s conclusion was the possibility to use an electrical distributed propulsion concept to reduce embedded storage energy [1]. Indeed, the implementation of many small engines allows to optimize the required propulsive energy with respect to safety requirements (for example engine failure during take off). In 2014, ONERA’s engineers have described and validated an AEA concept for a small business aircraft [2] [3] (Fig.1). To design an efficient electrical network, a multi-physic approach has been adopted taking into account flight performance, safety and environmental requirements, and lead to a safe and optimized advanced power architecture described in the following sections. Fig. 1: ONERA’s “concept plane”. Main characteristics of the “concept plane” Starting from a small aircraft main operational characteristics compliant with the CS23 regulations (4 PAX, 400 km range, 2h of autonomy,) an iterative multi-physic analysis, lead to the following aircraft design characteristics [2]: • The propulsion is performed through 40 Electric Ducted Fans (EDF) located along the wing span (20 on each side) in order to increase aerodynamic lift [2] • The propulsive force is also used to mainly control yaw effect allowing stabilizers size reduction and thus, aircraft weight reduction. However, it requires to combine EDFs properly to ensure control laws (lateral stability). • To be compliant with the CS23 certification rules and more particularly during the climb step in case of engine failure. • The different scenarios evaluated through multi-physic simulations show that the Polymer Exchange Membrane (PEM) hydrogen fuel cell concept providing the electric power (propulsive and systems) is the best solution. • No environmental control system is required due to flight characteristics (altitude < 3000 m). Propulsive architecture The main objective of this section is to explain the process used to define the optimal propulsive architecture which would be compliant with the CS23 certification rules but also optimised to control the yaw effect during flight. Flight performance analysis concerning the yaw effect control shows that the propulsive force must be controlled symmetrically at the same time on both wings and also on each wing in case of EDF failure. A second analysis concerning engine loss during the climb step shows that, to be compliant with the CS23 requirements, only 24 operational EDF are necessary. Starting from these assumptions, an advanced propulsive architecture has been identified consisting in 10 clusters of 2*2 EDF as shown on Fig. 2 (one colors for each cluster). Moreover, each EDF is controlled individually which allows reconfiguration of the overall electric propulsion system. Fig. 2a: Advanced propulsive architecture. Fig. 2b: zoom on the left wing (symmetrical system). Power architecture The next step was to design a safe and optimized advanced power architecture (Fig. 3). Aiming to reduce the common failure points, each EDF cluster is powered by its own fuel cell system (PEMFC and high pressure hydrogen tank) and its own electrical wiring interconnection systems (EWIS). To reduce the power architecture weight, a HVDC voltage (135 VDC) has been chosen for electrical power distribution, limiting to the number of conversion steps between fuel cell (available output level) and EDF. As a result, a system based on 10 primary power distributions has been selected, in which each primary distribution is composed of Busbar, Solid State Power Controller (SSPC) modules allowing electrical load control remote and cable protection, cable harnesses and inverter (DC/AC) at EDF level. To supply the power to the others systems, two secondary distributions (called essential and non essential) are designed with a 28 VDC voltage (Fig. 4). Each of them is connected at one primary distribution via a SSPC module and a DC-DC converter (135 VDC to 28 VDC). One of these secondary distributions (essential one) is also connected to Li-ion battery system (autonomy 20 min) allowing to supply the power to the main bus if the two primary distributions are out of order. The two secondary distributions are linked by a Bus Tie Breaker (BTB) to transfer the electrical power between the no-essential and the essential buses. Fig. 3: advanced power architecture (primary distributions) – Partial view . Fig. 4: advanced power architecture (secondary distributions). Analysis of power/energy needs First, let's define the electric power which must be embedded for this configuration. Starting from the greatest propulsive force required during the take-off and considering electric motor (97%) and engine power unit (95%) efficiencies, an electric power of 400 kW is required. The assumption that this power is equally distributed on the EDFs leads to a rated power of 10 kW for each one. To evaluate the electric power necessary to all others consumers, the embedded power of the Cirrus SR22 (2.4 kW) has been used as a reference to which additional power, roughly 1.6 kW, has been added for the new on-board systems. Taking into account the redundancy of the power system, an additional capacity of 8 kW has been taken into account for the hydrogen fuel cells. Let's now define the energy which has to be stored to perform a flight. For a range of 400 km, the consumed energy has been estimated with the following assumptions: 17 min for climb step (70 km), 78 min (330 km) for cruise and the descent steps and, roughly, 15 min for the safety reserve. To perform this flight 400 kWh are necessary for the propulsion which leads, considering the others consumers, to a total energy of 500 kWh. Components evaluation Now, we have to evaluate the main characteristics of each component of our proposed advanced power architecture. First, each fuel cell unit system is designed to provide a power of 40 kW (to supply 4 EDF, 10 kW each), excepted two fuel cell systems (44 kW) to supply equipments. To reduce the PEM fuel cell weight (and volume) and preserve the power capacity at different flight levels (from ground up to 3000 m), the inlet pressure has been set to 2 bars. The increasing of fuel cell pressure allows to increase its rated power [4]. This 2-bar pressure rate results from the use of two air compressors In these conditions and with a fuel cell efficiency of 0.6 a power density of 1.8 kW/kg is assumed. Thus, the gross weight of the PEM fuel cell system (i.e. 10 fuel cells with their hydrogen tanks, associated with air compressors) is about 226 kg and its volume reaches 1 m3 . For an embedded energy of 500 kWh, it is necessary to store 30 kg of hydrogen (at 700 bars) in 10 carbon fiber composite (CFC) material tanks of about 25,5 kg and 0.051 m3 each (with a diameter and length of 0.3 m and 1.63 m respectively). Second, each EDF supply chain is protected by a SSPC module of 10 kW under a HVDC voltage of 135 VDC. Based on the assumption that commercial SSPC modules will be used, a power density of 15 kW/kg has been estimated from data sheets, leading to a SSPC weight of about 27 kg. From the aircraft geometry defined by CAD, the power harnesses length of the propulsive part is close to 114 m. The environmental flight conditions (altitude) and installation characteristics analysis, give a wire gauge of "8" with an average linear weight of 0.130 kg/m. The corresponding cables weight is about 39 kg including their installation penalties (estimated at 30% of the cable weight). The overall weight of (DC/AC) inverters located in front of EDF is about 100 kg with a power density close to 4 kW/kg. Finally, the assumption that a power density of 5 kW/kg was available at EDF level has been done and thus the EDF system weight about 80 kg. The total weight of advanced power architecture has been summarized In the below table. Table 1: Summarize of weight of advanced power architecture. Conclusions In this paper, an advanced power architecture has been defined and pre-sized to demonstrate the viability of a AEA concept for a small business aircraft. The different analyses show that in the coming years, all necessary technologies will be available to enable the manufacturing of this concept plane. Moreover, our conceptual approach has shown that larger aircraft are feasible [5] but requires breakdown technologies for the Electric Ducted Fans and fuel cell systems. Acknowledgment Authors want to acknowledge the CEAtech engineers and the ONERA/CEA experts for their respective contributions. References 1 Rapport de synthèse du Groupe de Travail Études Prospectives “GTEP12”,, Onera – CEAtech, Septembre 2013 2 J. Hermetz and al, “L’avion d’affaire personnel à propulsion électrique, un « concept plane » “, workshop on « aviation à propulsion électrique », Aéro-club de France, April 2014. 3 J. Hermetz and al, « Electric Distributed Propulsion for Small Business Aircraft – A Concept-Plane » , MEA 2015 , February 2015. 4 F. Barbir, PEM Fuel Cells: Theory and Practice, Elsevier, 2013, 509 p. 5 C. Döll, B. Paluch, A. Guigon, D. Fraboulet; "Conceptual Feasibility Study for A Fully Electrically Powered Regional Transport Aircraft", 29th Congress of the International Council of the Aeronautical Sciences ICAS, Saint Petersburg, Russia, 7-12 September, 2014.